Fluorescence microscopy characterization of the bonding interface of scrimbers made of alkali-treated finely fluffed poplar veneers

To improve the penetration of adhesives on finely fluffed poplar veneer (FFPV), a 3% NaOH solution was used to treat FFPV heartwood and sapwood. The treated FFPVs were made into engineered wood composites, otherwise known as scrimbers, with phenol-formaldehyde (PF) resin. The static contact angle and the resin weight gain rate of the FFPV were tested. The shear strength of the FFPV scrimbers was investigated, and the bonding interface was characterized by fluorescence microscopy. The results revealed that the static contact angle of the treated FFPV decreased, but the resin weight gain rate increased. The shear strength of the treated FFPV scrimbers was lower than that of the untreated scrimbers. Fluorescence microscopy revealed that the bondline morphology of the treated FFPV scrimbers changed, with a thinner bondline, a deeper penetration distance, and a smaller glue stain. This result illustrated that alkali treatment can enhance the penetration of PF resin on the FFPV effectively, while excessive penetration of the adhesive should be avoided to ensure adequate bonding properties of the scrimber.